Apple Scab

Apple scab is a common and damaging fungal disease affecting apple and crabapple trees. Caused by the pathogen Venturia inaequalis, apple scab can significantly impact fruit quality and yield, as well as the overall health of the tree.

Symptoms

1. Leaf Spots: The most characteristic symptom is the appearance of dark, olive-green to black spots on the leaves. These spots are often sunken and can merge to form larger lesions. As the disease progresses, the affected leaves may become yellowed and fall off prematurely.
2. Fruit Scabs: On fruit, apple scab appears as dark, sunken lesions that can deform the fruit and make it unappealing. The lesions can also cause fruit to crack and rot, reducing its marketability and quality.
3. Twig and Branch Lesions: In severe cases, the disease can cause dark, sunken lesions on twigs and branches, which may lead to dieback.

Disease Cycle

1. Overwintering: The fungus overwinters in fallen leaves and in infected buds on the tree. In the spring, the fungus produces new spores that are released into the air.
2. Spore Release and Infection: The spores are spread by rain, wind, and splashing water. They infect new leaves and fruit, especially during wet conditions, which favor spore germination and infection.
3. Spread: Once the spores land on susceptible plant parts, they germinate and penetrate the tissue, causing the characteristic symptoms of apple scab. Infected plant tissues produce new spores, which spread to other parts of the tree and nearby plants.

Management

1. Cultural Practices
   • Sanitation: Remove and dispose of fallen leaves and other plant debris to reduce the source of overwintering spores. Clean up and destroy infected fruit and leaves.
   • Pruning: Prune trees to improve air circulation and reduce humidity around the foliage. This helps to minimize conditions favorable for fungal growth.
   • Proper Spacing: Space trees adequately to allow for good air circulation and sunlight, which helps reduce the moisture that fosters fungal growth.
2. Disease-resistant Varieties: Plant apple varieties that are resistant or less susceptible to apple scab. Many modern apple cultivars have been bred for resistance to this disease.
3. Fungicide Application
   • Preventive Sprays: Apply fungicides preventively, starting early in the growing season, particularly during periods of wet weather when the risk of infection is higher. Products containing captan, chlorothalonil, or other approved fungicides are commonly used.
   • Follow Instructions: Always follow the manufacturer’s instructions for application rates and timing to ensure effectiveness and minimize harm to the plant and environment.